The invention relates to a thermo-transfer ribbon with a customary carrier and thermo-transferable layers arranged thereon.
Thermo-transfer ribbons have been known for some time. They have a foil-like carrier, made, for example, of paper, of synthetic material or similar, a thermo-transfer color, specifically in the form of a synthetic material and/or wax-bonded dye or carbon black layer. In thermo-print technology, the thermo-transfer color is softened by means of a thermo-print head and transferred to a registration paper or a printing paper. Thermal printers or thermal print heads which can be used for this process are known, for example, from DE-AS 24 06 613 and from DE-OS 32 24 445. The following specific procedure can, for example, be followed. On the thermal print head of the printer a letter is formed which consists of heated points and is to be printed on a piece of paper. The thermal print head prints the thermo-transfer ribbon on an imprintable paper. The heated letter of the thermal print head, at a temperature of approximately 400.degree. C. results in softening of the thermo-transfer color at the heated spot and transfer onto the piece of paper in contact therewith. The used part of the thermo-transfer ribbon is then passed to a spool.
The thermo-transfer ribbon can have several thermo-transfer colors next to each other. With the combination of the basic colors; blue, yellow, red, colored printed images can be produced. In contrast to the customary color photography, detrimental developing and fixation is eliminated. Thermo printers can be operated at high speed and without interfering ambient noise. For example, a DIN A4 size page can be printed in about 10 seconds.
Serial printers or line printers can be used for printing. The serial printers operate with a relatively small, movable print head of up to approximately 1 cm.sup.2. On the print head are arranged, vertically vis-a-vis the writing direction, 1 or 2 rows of dots (dot=addressable heating point). The dot-diameter is between approximately 0.05 and 0.25 mm. The number of dots per dot row is between 6 and 64, which corresponds to a resolution ranging from 2 to 16 dots/mm. Higher resolutions, for example, 24 to 32 dots/mm can be expected in the near future. It is typical with respect to the serial thermo head that it is moved during the printing process horizontally toward the transport direction of the paper. In contrast to the serial print head, a line print head is a stationary head or strip. Since the print strip is not movable, it must span the width of the substrate which is to be printed. Print strips are offered at lengths of up to 297 mm. Resolution and dot-size correspond to those of serial heads. The serial printers are used in typewriters, video print outs, in PC applications and for word processors, while line printers are used specifically in bar code graph printers, computer units with high output volume data, in fax machine applications, ticket printers, address printers, color copiers and the CAD/CAM system.
In addition to the above described thermo-transfer ribbons, there are also thermo-transfer ribbons where the thermal symbol is not imprinted through action of a thermal printer, but by means of resistance heating of the specifically designed foil-like carrier. The resistance heating occurs in that the thermo-transfer color, and/or its carrier contain electrically conductive materials. The thermo-transfer color, which is the "functional layer" during the printing process, contains, in addition, the materials already described above. This is also called an ETR material (electro thermal ribbon material). A corresponding thermo-transfer print systems is described, for example, in U.S. Pat. No. 4,309,117.
In the above described systems of thermo-transfer ribbons, the sharpness of the print and the optical density of the produced print depends, among others, upon the adhesion to the paper of the thermo-transfer color. It is proportional to the adhesive surface of the adhesive power. Rough paper has low adhesive surface, since only the raised portions of the paper surface are wetted by the melted thermo-transfer color. For that reason, in DE-A-35 07 097 there is formed on the layer of the thermo-transfer color a so-called "filling layer", which consists of a in melted state low viscosity material, which flows, during the printing process, into the valleys of the rough paper surface and thus increases the adhesive surface. It is of disadvantage here that the molten filling layer, in case of very smooth paper, having a roughness of no more than 200 Bekk, is no longer able to penetrate the paper during the printing process, so that a layer remains between paper surface and color layer. The layer, therefore, has the effect of a hold-off layer, as described in EP-A-O 042 954. The hold-off layer, however, results in insufficient document color fastness, since it prevents the penetration of the thermo-transfer color into the paper. For a document color fastness layer, a hold-off layer is undesirable.
In order to avoid the above indicated drawbacks of the state of the art, specifically to beneficially facilitate the writing on rough as well as smooth paper, EP-B-0-348 661 suggests to incorporate into the hold-off layer or adhesive layer, also called top coat, a tackifying, in finely dispersed form in paraffin embedded hydrocarbon, whereby the paraffin has a melting point of 60 to 95.degree. C. The teaching according to EP 0 206 036 attempts to avoid the necessity of such adhesive layer or top coat, in that a wax layer is formed on the layer of the synthetic material-bonded thermo-transfer color and the synthetic material-bonded thermo-transfer color contains a thermoplastic synthetic material with a softening point of 60 to 140.degree. C.
The above described thermo-transfer ribbons gain more and more access in high-speed printers, specifically in industrial application, whereby in this case a so-called "real edge" or "corner-type" print head is going to be used. With these print heads, the row of dots is arranged on a ceramic substrate, close to the edge or directly on the edge. The advantage of the edge-type head lies in shorter cooling down times and, consequently, higher writing frequency. Print speeds of from 3 to 12" per second can be obtained with this type of head. Special requirements exist as to print quality with respect to prints obtained with high speed print, particularly high speed color print, i.e. requirements regarding edge sharpness, resolution and optical density. One particular field of application is the printing of paper and plastic labels. As to the latter, high scratch-resistance of prints is desired.
The thermo-color ribbons provided up to now as state of the art for high speed printer application do not fully meet the mentioned requirements.
The invention was therefore based on the object of making available a thermo-transfer ribbon with which high printing speeds and satisfactory print quality can be obtained. The thermo-transfer color transferred during printing is to demonstrate excellent adhesion and excellent scratch-resistance, specifically on paper and plastic labels.